Thanks, but I don't know how these things apply to subroutine signatures.
For instance how does the XS interface hinder the implementation of proper signatures?
Well, I was talking in general terms about why perl5 doesn't tend to get new features, in response to your general compliant about the management of perl5.
But specifically for signatures, there are many things that can break in XS. For example, if we introduce new op types to handle signature processing, then XS modules which manipulate the optree, or which assume certain ops at the start of a sub, or which modify op behaviour by changing their pp_addr function pointer, may all break. Or modules which use keyword plugins and make certain assumptions about allowed syntax. Or XS code which directly manipulates @_. Or code which attempts to hook into subroutine entry and exit. Or profilers that think they know all valid op types.
There are many things that can break just that I can think of; when deployed, there will be other things that break that hadn't even occurred to me.